Disable FMul/FDiv cross-interaction and GammaF select absorption rules

FDiv-to-FMul constant chain rules create feedback loop: new FMul nodes
from FDiv(FMul(x,c1),c2)→FMul(x,c1/c2) feed back into FMul chain merge,
which creates more FMul nodes that re-enter FDiv cross rules. 252K matches
at iter 7 on mouse-shader with exponential growth. Pure FMul and FDiv
constant chains are preserved.

Also disables GammaF FMul select absorption which injects 599 new FMul
nodes per iteration into the chain merging pipeline.

Author: LegNeato

rust/spirv-tools-opt/src/rules/floating_point.egg

@@ -290,14 +290,19 @@
       ((union e (FDiv x (FConst (* a b))))))
 
 ; (x * a) / b = x * (a / b)
-(rule ((= e (FDiv (FMul x (FConst a)) (FConst b)))
-       (!= b 0.0))
-      ((union e (FMul x (FConst (/ a b))))))
+; DISABLED: FDiv→FMul cross-interaction creates feedback loop with FMul chain merging.
+; New FMul nodes from these rules feed back into FMul(FMul(x,c1),c2) chain merge,
+; which creates more FMul nodes that re-enter FDiv cross rules. 252K matches at iter 7
+; on mouse-shader with exponential growth. Pure FMul and FDiv chains are kept.
+; (rule ((= e (FDiv (FMul x (FConst a)) (FConst b)))
+;        (!= b 0.0))
+;       ((union e (FMul x (FConst (/ a b))))))
 
 ; (x / a) * b = x * (b / a)
-(rule ((= e (FMul (FDiv x (FConst a)) (FConst b)))
-       (!= a 0.0))
-      ((union e (FMul x (FConst (/ b a))))))
+; DISABLED: Same feedback loop as above. 73K matches at iter 7 on mouse-shader.
+; (rule ((= e (FMul (FDiv x (FConst a)) (FConst b)))
+;        (!= a 0.0))
+;       ((union e (FMul x (FConst (/ b a))))))
 
 ; (x + a) + b = x + (a + b)
 (rule ((= e (FAdd (FAdd x (FConst a)) (FConst b))))

rust/spirv-tools-opt/src/rules/rvsdg.egg

@@ -700,10 +700,13 @@
 (rule ((= e (GammaI c x (Mul x y))))
       ((union e (Mul x (GammaI c (Const 1) y)))))
 ; Float ops use GammaF and FConst
-(rule ((= e (GammaF c (FMul x y) x)))
-      ((union e (FMul x (GammaF c y (FConst 1.0))))))
-(rule ((= e (GammaF c x (FMul x y))))
-      ((union e (FMul x (GammaF c (FConst 1.0) y)))))
+; DISABLED: Creates 599 new FMul nodes that feed into FMul constant chain merging,
+; amplifying the exponential growth. This is a special case of the GammaF FMul
+; hoisting that was already disabled above (lines 428-429).
+; (rule ((= e (GammaF c (FMul x y) x)))
+;       ((union e (FMul x (GammaF c y (FConst 1.0))))))
+; (rule ((= e (GammaF c x (FMul x y))))
+;       ((union e (FMul x (GammaF c (FConst 1.0) y)))))
 
 ; Nested Gamma flattening (when inner condition relates to outer)
 ; select(c, select(c, a, b), d) = select(c, a, d) - already have